Image graphics are omnipresent in modern life. Images and data that warn, educate, entertain, advertise, etc. are applied on a variety of interior and exterior, vertical and horizontal surfaces. Non-limiting examples of image graphics range from advertisements on walls or sides of trucks, to posters that advertise the arrival of a new movie, warning signs near the edges of stairways, and the like.
The use of thermal and piezo inkjet inks has greatly increased in recent years with accelerated development of inexpensive and efficient inkjet printers, ink delivery systems, and the like.
Thermal inkjet hardware is commercially available from a number of multinational companies, including without limitation, Hewlett-Packard Corporation of Palo Alto, Calif.; Corporation of San Diego, Calif.; Xerox Corporation of Rochester, N.Y.; ColorSpan Corporation of Eden Prairie, Minn.; and Mimaki Engineering Co., Ltd. of Tokyo, Japan. The number and variety of printers change rapidly as printer makers are constantly improving their products for consumers. Printers are made both in desk-top size and wide format size depending on the size of the finished image graphic desired. Non-limiting examples of popular commercial scale thermal inkjet printers are Encad Corporation's NOVAJET Pro printers and Hewlett-Packard Corporation's 650C, 750C, and 2500CP printers. Non-limiting examples of popular wide format thermal inkjet printers include Hewlett-Packard Corporation's DesignJet printers, where the 2500CP is preferred because it has 600×600 dots/inch (dpi) resolution with a drop size in the vicinity of about 20 picoliters (pL).
3M Company, of St. Paul, Minn., markets Graphic Maker Inkjet software useful in converting digital images from the Internet, ClipArt, or Digital Camera sources into signals to thermal inkjet printers to print such image graphics.
Inkjet inks are also commercially available from a number of multinational companies, particularly 3M Company which markets its Series 8551; 8552; 8553; and 8554 pigmented inkjet inks. The use of four process colors: cyan, magenta, yellow, and black (generally abbreviated “CMYK”) permit the formation of as many as 256 colors or more in the digital image.
Media for inkjet printers are also undergoing accelerated development. Because inkjet imaging techniques have become vastly popular in commercial and consumer applications, the ability to use a personal computer to print a color image on paper or other receptor media has extended from dye-based inks to pigment-based inks. The media must accommodate that change. Pigment-based inks provide more durable images because of the large size of colorant as compared to dye molecules.
Inkjet printers have come into general use for wide-format electronic printing for applications, such as engineering and architectural drawings. Because of the simplicity of operation and economy of inkjet printers, this image process holds a superior growth potential promise for the printing industry to produce wide format, image on demand, presentation quality graphics.
Therefore, the components of an inkjet system used for making graphics can be grouped into three major categories:                1. Computer, software, printer        2. Ink        3. Receptor medium        
The computer, software, and printer will control the size, number and placement of the ink drops and will transport the receptor medium through the printer. The ink will contain the colorant that forms the image and carrier for that colorant. The receptor medium provides the repository that accepts and holds the ink. The quality of the inkjet image is a function of the total system. However, the compositions and interaction between the ink and receptor medium are most important in an inkjet system.
Image quality is what the viewing public and paying customers will want and demand to see. From the producer of the image graphic, many other obscure demands are also placed on the inkjet media/ink system from the print shop. Also, exposure to the environment can place additional demands on the media and ink (depending on the application of the graphic).
Current inkjet receptor media, direct coated with compositions according to the disclosure contained in U.S. Pat. No. 5,747,148 (Warner et al.) and in PCT Patent Publication Nos. WO 99/07558 (Warner et al.) and WO 99/03685 (Waller et al.), are marketed by 3M Company under the brands 3M™ Scotchcal™ Opaque Imaging Media 3657-10 and 3M™ Scotchcal™ Translucent Imaging Media 3637-20, 8522, and 8544, respectively. Another inkjet receptor media is disclosed in coassigned PCT Patent Publication No. WO 97/33758 (Steelman et al.) which combines a hygroscopic layer on a hydrophilic microporous media.
Inkjet inks are typically wholly or partially water-based, such as disclosed in U.S. Pat. No. 5,271,765. Typical receptors for these inks are plain papers or preferably specialty inkjet receptive papers which are treated or coated to improve their receptor properties or the quality of the images resulting therefrom, such as disclosed in U.S. Pat. No. 5,213,873.
Many inkjet receptor compositions suitable for coating onto plastics to make them inkjet receptive have been disclosed. Typically, these receptor layers are composed of mixtures of water-soluble polymers which can absorb the aqueous mixture which the inkjet ink comprises. Very common are hydrophilic layers comprising poly(vinyl pyrrolidone) or poly(vinyl alcohol), as exemplified by U.S. Pat. Nos. 4,379,804; 4,903,041; and 4,904,519. Also known are methods of crosslinking hydrophilic polymers in the receptor layers as disclosed in U.S. Pat. Nos. 4,649,064; 5,141,797; 5,023,129; 5,208,092; and 5,212,008. Other coating compositions contain water-absorbing particulates, such as inorganic oxides, as disclosed in U.S. Pat. Nos. 5,084,338; 5,023,129; and 5,002,825. Similar properties are found for inkjet paper receptor coatings, which also contain particulates, such as cornstarch as disclosed in U.S. Pat. Nos. 4,935,307 and 5,302,437.
The disadvantage that many of these types of inkjet receptor media suffer for image graphics is that they comprise water-sensitive polymer layers. Even if subsequently overlaminated, they still contain a water-soluble or water-swellable layer. This water-sensitive layer can be subject over time to extraction with water and can lead to damage of the graphic and liftoff of the overlaminate. Additionally, some of the common constituents of these hydrophilic coatings contain water-soluble polymers not ideally suitable to the heat and UV exposures experienced in exterior environments, thus limiting their exterior durability. Finally, the drying rate after printing of these materials appears slow since until dry, the coating is plasticized or even partially dissolved by the ink solvents (mainly water) so that the image can be easily damaged and can be tacky before it is dry.
In recent years, increasing interest has been shown in microporous films as inkjet receptors to address some or all of the above disadvantages. Both Warner et al. and Waller et al. publications and Steelman et al. application, identified above, disclose microporous films to advantage. If the film is absorbent to the ink, after printing the ink absorbs into the film itself into the pores by capillary action and feels dry very quickly because the ink is away from the surface of the printed graphic. The film need not necessarily contain water-soluble or water-swellable polymers, so potentially could be heat and UV resistant and need not be subject to water damage.
Porous films are not necessarily receptive to water-based inkjet if the material is inherently hydrophobic and methods of making them hydrophilic have been exemplified, for example, by PCT Patent Publication No. WO 92/07899.
Other films are inherently aqueous ink absorptive because of the film material, e.g., Teslin™ (a silica-filled polyolefin microporous film) available from PPG Industries and of the type exemplified in U.S. Pat. No. 4,861,644. Possible issues with this type of material are that if used with dye-based inks image density can be low depending on how much of the colorant remains inside the pores after drying. One way of avoiding this is to fuse the film following printing as exemplified in PCT Patent Publication No. WO 92/07899.
Other methods are to coat the microporous film with a receptor layer as disclosed in PCT Patent Publication No. WO 97/33758 (Steelman et al.) and U.S. Pat. No. 5,605,750.
As stated above, the relationship between the ink and the media is key to image graphic quality. With printers now reaching 1400×720 dpi precision, inkjet drop size is smaller than in the past. As stated previously, a typical drop size for this dpi precision, is about 20 picoliters, which is a fraction of the size of prior drop sizes of 140 picoliters used in wide format inkjet printers, most notably and commonly Encad™ NOVAJET III, IV, and Pro models. Some printer makers are striving for even smaller drop sizes, while other printer makers are content with the larger drop sizes for large format graphics. With pigmented inkjet inks, drop size determines the quantity of pigment particles that reside in each drop and are to be directed to a predetermined area of media.
When the inkjet ink drop contacts the receptor medium, a combination of two things occurs. The inkjet drop diffuses vertically into the medium and diffuses horizontally along the receptor surface, with a resulting spread of the dot.
However, with pigment-based inkjet inks of the right particle size and if used with a film of the right pore-size, some filtration of the colorant is possible at the surface of the film resulting in a good density and color saturation. However, images can still be very poor if dot-gain is low due to “banding phenomena” where insufficient ink remains to generate the appropriate halftone image. If dot-size is too small, then errors due to media advancement or failed printhead nozzles can cause banding. This problem would not be seen with larger drop size printers because larger dots could cover up prior printing errors. However, if dots are too large, then edge acuity is lost. Edge acuity is a reason for increased dpi image precision. Ability to control dot diameter is therefore an important property in an inkjet receptor medium.
U.S. Pat. No. 5,605,750 exemplifies a pseudo-boehmite coating applied to the silica-filled microporous film, such as Teslin™. The coating contains alumina particles of pseudo-boehmite of pore radius 10 to 80 Å. Also disclosed is an additional protective layer of hydroxypropylmethyl cellulose.
Several problems exist using receptor coatings mentioned above. The rate of ink absorption is, at most for a water-swellable coating, 8-10 ml/sec/M2; this is slow when compared to the rate of ink drop application. Secondly, the volumes of ink applied by many popular wide format inkjet printers at 140 pL/drop (HP 2500: 20 pL/drop but 160 pL/dot) can create problems, such as “feathering”, “demixing”, and coalescence of the ink.
Because most of the printers in the office of today use water-based inks, when those printers and inks are used on conventional ink jet films, a lot of ink must be laid down to give suitable image density for transmitted images. When the ink is laid down rapidly on known films, such as those identified above, the ink tends to suffer from uncontrolled coalescence, or uneven gathering of the ink drops, resulting in streaking, banding and blotching. In order to avoid coalescence and give acceptable image quality, printers use a “transparency” mode which delivers the ink slowly over multiple passes. This results in much slower overall print speed and is a major source of dissatisfaction with making overhead transparencies on ink jet printers.
Coassigned PCT Patent Publication No. WO 99/55537 (Ylitalo et al.) “Microembossed Receptor Media”, discloses the use of microembossed films for receiving and displaying images delivered by ink jet printers of various kinds, including those using water-based inks, with patterns with cavity size in the range of 20-1000 pL. This range was chosen to encompass the range of ink volumes of known printers, so that the number of cavities per area, or cavity density, was equal to or greater than the resolution or dots/inch (“dpi”) of the printer.